Various types of transducer assemblies are known. One type, shown in U.S. Pat. No. 5,142,260 to William N. House, is illustrated in FIG. 1. This assembly includes a magnetic circuit structure 10 with two aligned magnetic discs 12, 14, which are axially polarized and oriented so their resultant flux fields oppose one another. A spacer 16 of either ferrous or nonferrous material is sandwiched between the magnets 12, 14 to help control the magnetic field characteristics. As a result of the opposing axial alignment, magnetic flux lines 18 emanate from the magnetic poles 20, 22 that face each other and are focused and directed radially outward from the region 24 between the magnets 12, 14.
This prior art structure serves two functions—to increase the number of flux lines per unit cross-sectional area in the region adjacent the structure's outer surface 26, and to direct the flux lines 18 on paths essentially perpendicular to the axis 28 of the structure. Ideally, all flux lines 18 emanating from the structure 10 would be in directions perpendicular to the structure's axis 28 to maximize the force on a cylinder conductor 30 throughout its axial length. However, as stated in the '260 patent, regions of flux lines exist that are not perpendicular to the structure's axis. If a current carrying conductor 30 moves in an axial direction from a center region A to a magnet's center region C, the instantaneous force on the conductor 30, in the direction parallel to the axis 28, decreases as a function of the angle to zero. This leads to the phenomenon described in the '260 patent termed “field reversal”. Field reversal is typically one of the restrictions encountered with returnless path structures, such as structure 10 in FIG. 1. Thus, motion of coil 30 in a linear direction will generally occur only within a relatively small portion of the axial length.
U.S. Pat. No. 5,142,260 addresses these problems by sandwiching one or more additional radial magnets and/or spacers between the opposing magnets of the prior art assemblies. The radial magnets' outer poles have the same polarity as the magnets' facing poles. Flux lines emanating from the radial magnets are opposed by the fields of the axial magnets and directed outward on a path perpendicular to the structure's axis. The radial magnets' flux lines travel outward and around to the opposite polarity poles of the axial magnets. According to the '260 inventor, this increases the total flux lines provided by the structure.
The devices described in U.S. Pat. No. 5,142,260 still have a number of drawbacks. It appears that the '260 inventor sought to increase coil performance by lengthening the coil/magnetic field interaction distance and by ensuring that the drive coil wire was as close to the magnet assembly as possible. In doing so, the coil and the outward magnetic field flux lines were both lengthened. These steps increase the weight and complexity of the magnetic system and further cause an increase in the force on the coil, which leads to nonlinearity in the coil's response to the magnetic field as the coil oscillates during its excursion. In general, such systems are inefficient because they require many windings in order for their drive cylinder to effectively interact with the increased magnetic field size. In addition, they tend to overheat and/or require cooling, and they tend to develop various signal distortions during use. In sum, the above problems cause the '260 devices to be inefficient, heavy, and expensive to manufacture. In addition, extensive periods of zero or reverse force on the coil persist at pole reversal.
To address some of the above drawbacks, designers have attempted to restrict excursion distance and design a relatively flat transducer. For example, U.S. Pat. No. 5,668,886 to Sakamoto et al. describes a loudspeaker having two magnets with like poles facing one another and held in place by opposed frame members. A ferrous center plate is interposed between the two magnets. A cylindrical voice coil is positioned about the magnets and center plate. A diaphragm is mounted laterally about the exterior periphery of the voice coil. The '886 device appears similar to conventional dynamic transducers, except that a ferrous center plate is used to direct a single radially emanating magnetic field outward. The '886 device is still relatively heavy and still requires a tall, cylindrical drive coil to interact with the magnetic field at close range.
The device of U.S. Pat. No. 5,764,784 to, Sato et al. describes a single disk magnet fixed to an inner surface of a flat casing. Within the casing, a diaphragm is spaced apart from the magnet. A relatively short, hollow, cylindrical drive coil is coaxial with the magnet and is fixed to an opposed face of the diaphragm. According to the '784 inventors, this arrangement has diminished power consumption, reduced thickness, and greater efficiency. The '784 device suffers, however, in that the diaphragm is limited in the distance it can travel during use, since it will rapidly hit the magnet face if the input signal is of sufficient strength. In addition, the '784 drive coil is not entirely immersed in a symmetric field throughout its excursion travel. This can affect signal purity and can be a source of signal distortion.
U.S. Pat. No. 5,905,805 to Hansen describes a transducer with a circular center diaphragm. A flat planar drive coil is formed on one surface of the diaphragm. A pair of opposed cylinder magnets is provided, one magnet being spaced apart from each side surface of the diaphragm. The magnets are in a repulsing configuration, with like pole faces oriented toward one another. This produces a radially emanating magnetic field. As in the '784 device, though, the '805 device is limited in the distance the diaphragm can travel before it hits one of the cylinder magnets. This design also suffers from the relatively minor amount of interaction between the planar coil windings and the radially emanating magnetic field.
Thus, a need yet exists for a transducer that avoids the pitfalls described above. Ideally, such a transducer would be short in height and light in weight. Its drive coil would interact efficiently with the magnetic field and produce greater linearity of response throughout a large frequency spectrum. In addition, such an improved transducer would not overheat or require cooling, thus maintaining a much higher and more consistent power output throughout its operation. Similarly, more power output per weight of the device would be available as well as greater excursion distance of the coil.